1. Field of the Invention
The present invention relates to a load driving circuit such as a motor driver and, more particularly, to a load driving circuit having a current detection function for detecting a driving current with a current mirror arrangement.
2. Description of the Related Art
FIG. 8 is a block diagram of a motor and a driving circuit according to the related art. FIG. 8 shows transistors Tr 801–811, a power supply voltage VM, a motor 819, a current detection amplifier 821 and a detection resistor 823 for detecting a driving current Io. A dotted line in the figure indicates the driving current Io flowing through Tr 805→a V-phase driving terminal→W-phase driving terminal→Tr 811.
Referring to FIG. 8, a node C between an end of the detection resistor 823 and the transistors Tr 803–811 is connected to an inverting input terminal of the current detection amplifier 821. The other end of the detection resistor 823 is grounded. A non-inverting input terminal of the current amplifier 821 is grounded. An output terminal of the current detection amplifier 821 is connected to a control circuit (not shown). The control circuit controls a gate voltage of the transistors Tr 801–811 in accordance with an output voltage of the current detection amplifier 821.
More specifically, when the driving current Io varies and a voltage at the node C varies accordingly, the output voltage of the current detection amplifier 821 varies. When detecting that the driving current Io grows to an excess current, the control circuit (not shown) controls the current by controlling the gate voltage of the transistors Tr 801–811, in accordance with the output voltage of the current detection amplifier 821.
The driving current Io is controlled according to the related art in a procedure as described above. Employing a circuit construction as shown in FIG. 8, however, presents a problem in that the voltage generated across the detection resistor 823 is counted as a loss of the power supply voltage VM.
One method to address the above problem is a current detection circuit in which one of two n-channel MOSFET transistors connected in parallel is used as an output transistor and the other is used as a detection transistor (see FIG. 1 of patent document No. 1). By using this current detection circuit, it is possible to eliminate a detection resistor from a path of a driving current connecting a power supply with a power ground.
[Patent document No. 1] Japanese Patent No. 2570523
There is a problem, however, in that, for proper operation of a current detection circuit, the resistance of a detection resistor should be low, and the low resistance causes the detection of voltage to become difficult. The patent document No. 1 proposes a current detection circuit (FIG. 3) in which the driving current is not diverted into a power unit and a detection unit. In order to operate this current detection circuit, however, it is necessary to provide a constant-current source for detection.